i though E398 is not that hot ? 

You sure related to temperature?

First case tho

Use heatsink stick on the modem and use a USB fan

well warranty void or not , which i am not sure. this point u need to refer back your seller,.

but normally it should be fine blowing with a usb fan 

if the temperature is high, u need to tame it down first before u proceed to purchase antenna to plug in,

Just stick to the Plastic cover will do

E398 already got thermal tape inside to transfer the heat from the Chipset IC to the chassis

BTW I have to warn anyone who attempt to diassemble any RF device including smartphone, there are antenna around the chassis, make sure don't drop or bend or lost the PIN connecting to the antenna

What's wrong with USB bro

One of our user is using WR710N now
Might need custom firmware to work better

Buy quality one dont buy Cap Ayam cheapo bro

Original Transformer also burn?

Try finding lazada

Buy original Apple, wont so easy Hangus, their circuit design and protection very good

I got Huawei original, PM me once Hunkey hangus

I am curious mind I ask something, how much P.O. Box charge every month

How much is your signal?
My area is 35%~ and my modem was super hot.
After plug in the antenna, it goes to 80% and heat reduces 50%.

Never had my USB chargers burnt before *touch wood* 
Was using 2 x MR3020+E398 24x7 ever since I bought the the modem from blacktubi. I've supposed it's more than a year now.
One thing to note about using high power 3G/4G modem with MR3020. The power supplied via USB might not be sufficient, make sure you're using a decent USB charger (i'm using one that can supply at least 2A) or you can supply additional power via a self-powered USB hub or the Y-cable.

Just want to share one of the best USB wall charger that I've come across so far - PortaPow Quad USB
Main features that I like about it:
- Quad USB (a bit bulky but i don't mind )
- Auto lower individual USB power to the minimum if device is fully charged (with LED indicator)
- 3 Years warranty (not that it will be easier for us to claim since it's based in UK  )
You can buy one from eBay at around RM70.

For WR710N, I'm using one running on ROOter. Bought two and still undecided what to use with the remaining one. There are a couple of sellers selling it here. Just need to do a quick search in Networking Related Items.